1. Field of the Invention
The present invention relates to a semiconductor device having a TFT (thin film transistor) provided on an insulating substrate such as glass, and a method of manufacturing the semiconductor device.
2. Discussion of the Related Art
As the semiconductor having the TFT formed on the insulating substrate made of glass or the like, there have been known an active liquid crystal display device, an image sensor and the like, which use the TFT for driving a pixel.
A thin film silicon semiconductor is generally used for the TFT used in these devices. The thin film silicon semiconductor is roughly classified into the amorphous silicon semiconductor (a-Si) type and the crystalline silicon semiconductor type. The amorphous silicon semiconductor is most generally used because the manufacturing temperature is low, it can be relatively readily manufactured by a vapor phase method, and the mass productivity is sufficient. However, since the physical properties of the amorphous silicon semiconductor is inferior to the crystalline silicon semiconductor such as the electrical conductivity or the like, there is a strong demand to establish a method for manufacturing the TFT formed of the crystalline silicon semiconductor for the purpose of obtaining the higher speed characteristics in the future. As the crystalline silicon semiconductor, there have been known non-single crystalline silicon semiconductors such as polycrystalline silicon, microcrystalline silicon, amorphous silicon containing crystal components, semi-amorphous silicon having an intermediate state between the crystal property and the amorphous property, and the like. Hereinafter, the nonsingle crystalline silicon semiconductors having these crystal properties are called a crystalline silicon.
As a method of obtaining the thin film silicon semiconductors with these crystal properties, there have been known the following methods.
(1) A crystalline film is directly formed at the time of film formation.
(2) The energy of a laser illumination is applied to an amorphous semiconductor film which has been previously formed to provide the crystal property.
(3) A heat energy is applied to an amorphous semiconductor film which has been previously formed to provide the crystal property.
However, in the method (1), it is technically difficult to uniformly form a film having the excellent semiconductor physical properties all over the upper surface of a substrate. Further, since the film forming temperature is high, that is, 600xc2x0 C. or more, there rises such a problem in costs that an inexpensive glass substrate cannot be used. In the method (2), in the case of an example of an excimer laser which is most generally used now, there rises a problem that a through-put is low because a laser beam radiated area is small. Furthermore, the stability of the laser beam is insufficient to uniformly treat the entire upper surface of a large-area substrate, whereby it strongly seems as if this method is the technique for the coming generation. In the method (3), there is an advantage in that this method can cope with the large-area of the substrate in comparison with the methods (1) and (2). However, a high temperature of 600xc2x0 C. or more is required as a heating temperature, and taking the inexpensive glass substrate used into consideration, it is necessary to further decrease the heating temperature. In particular, the current liquid-crystal display unit advances to a large screen, and for that reason, it is necessary to use a large-scale glass substrate likewise. When such a large-scale glass substrate is used, there rises a serious problem that the contraction or strain of the substrate in the heating process essential to the semiconductor manufacture makes the accuracy in mask matching or the like deteriorate. In particular, in the case of the 7059 glass which is most generally used now, the temperature of the strain point is 593xc2x0 C., whereby the conventional heat crystallization method causes the substrate to be largely deformed. Moreover, in addition to the temperature problem, since the current process requires heating time of several tens hour or more necessary for crystallization, it is also necessary to shorten the heating time.
The present invention has been made to eliminate the above-mentioned problems, and an object of the invention is to provide a process of lowering a temperature necessary for crystallization and reducing a time therefor in a method of manufacturing a thin film formed of a crystalline silicon semiconductor by application of a method of crystallizing a thin film formed of an amorphous silicon by heating. The crystalline silicon semiconductor prepared by a process of the present invention has the physical properties not lower than those prepared by the conventional process, and applicable even to the active layer region of a TFT.
The inventors have conducted the following experiments in the above-mentioned method of forming an amorphous silicon semiconductor film by the CVD method or the sputtering method to crystallize the film thus formed by heating, and considered the experiment result.
First, the mechanism of forming the amorphous silicon film on a glass substrate to crystalize the film by heating has been investigated. As a result, it has been observed that the crystal growth started from an interface between the glass substrate and the amorphous silicon, then developed into the columnar shape perpendicular to the front surface of the substrate when it has the thickness of a certain degree.
It is considered that the above-mentioned phenomenon is caused by the fact that a crystalline nucleus forming a base of the crystal growth (the source forming a base of the crystal growth) exists in the interface between the glass substrate and the amorphous silicon film and the crystal grows from the nucleus. Such a crystalline nucleus is considered to be of a bit of impurity metallic element which exists on the surface of the substrate or the crystalline component of the glass surface (it is considered that the crystalline component of silicon oxide exists on the surface of the glass substrate as called the crystallized glass).
Therefore, it was considered that the temperature of crystallization can be lowered by more positively introducing the crystalline nucleus, and for the purpose of confirming this effect, a bit of other metals was formed on the substrate, and a thin film of the amorphous silicon was then formed thereon. Thereafter, the experiment of crystallization by heating was conducted. As a result, it was confirmed that, in the case of forming several metals on the substrate, the temperature of crystallization was lowered, and it was expected that there occurred crystal growth which had the foreign matter as the crystalline nucleus. Therefore, the mechanism of a plurality of impurity metals which could lower the temperature has been investigated in more detail.
The crystallization can be divided into two stages, that is, an initial nucleus creation and the crystal growth from the nucleus. The speed of the initial nucleus production was observed by measuring a time until fine crystals occurred in the dot pattern at a given temperature. That time was reduced in any cases of the thin films forming the above impurity metals, and the effect of lowering the temperature of crystallization when the crystalline nucleus was introduced was confirmed. Further, the growth of a crystal particle after nucleus production was investigated while changing the heating time. As a result, though this was beyond all expectations, it was observed that even the speed of crystal growth after the nucleus production was remarkably increased in the crystallization of the amorphous silicon thin film formed on the film of a certain metal. This is beyond all expectations. This mechanism will be described in more detail later.
In any case it was found that, as a result of the two effects mentioned above, when certain types of metals in trace amounts are used to form a film on which a thin film made of amorphous silicon is then formed and then heated for crystallization, sufficient crystallization is achieved at temperatures of 580xc2x0 C. and below for a period of about 4 hours, a fact which could not be foreseen according to the prior art. Examples of metal impurities having such an effect include iron, cobalt, nickel, copper, palladium, silver and platinum. All of these metals are often used as catalyst materials, and thus hereunder in the present specification they will be referred to as xe2x80x9cmetal catalyst for low temperature crystallizationxe2x80x9d. of these, the metal with the most notable effect and the easiest material to handle is nickel, and thus hereunder in the present specification nickel will be used as the center of discussion.
As an example of the degree of the effect provided by nickel, when on an untreated substrate (Corning 7059), i.e. one on which no thin film of trace nickel had been formed, a thin film of amorphous silicon formed by the plasma CVD method was heated in a nitrogen atmosphere for crystallization, a heating temperature of 600xc2x0 C. required a heating time of 10 hours or more. However, when an amorphous silicon thin film was used on a substrate on which had been formed a thin film of trace nickel, a similar crystalline state could be obtained with only about 4 hours of heating. Raman spectroscopy was used for the judgment of this crystallization. From this alone it is clear that the effect of nickel is exceptional.
As is clear from the above explanation, if an amorphous silicon thin film is formed on a thin film formed using a trace amount of a metal catalyst for low temperature crystallization, then it becomes possible to lower the crystallization temperature and shorten the time required for crystallization. Here, a more detailed explanation will be provided with the assumption that the process is used for the preparation of a TFT. The description will be more specific later, but the same effect is achieved not only when a thin film of a metal catalyst for low temperature crystallization is formed on the substrate, but also when it is formed on the amorphous silicon, and also with ion implantation, and therefore hereunder in the present specification all of these successive treatments will be called xe2x80x9caddition of a trace amount of a metal catalyst for low temperature crystallizationxe2x80x9d. As the metal catalyst for low temperature crystallization, it is useful to use at least one element selected from the group consisting of iron, cobalt, nickel, copper, palladium, silver and platinum, but based on our discovery the Group VIII elements Ru, Rh, Os and Ir may be mentioned as elements having the same effect as the above mentioned materials.
First an explanation will be given regarding a method of addition of the metal catalyst for low temperature crystallization. For the addition of a trace amount of the metal catalyst for low temperature crystallization, the same temperature lowering effect is provided either by a method of forming a thin film containing a trace amount of the metal catalyst for low temperature crystallization on a substrate and then forming thereon an amorphous silicon film; or by a method of first forming the amorphous silicon film and forming the thin film containing a trace amount of the metal catalyst for low temperature crystallization, and the formation of the films may be either by a sputtering method or a vapor deposition method, and thus the effect is accomplished regardless of the film-forming method.
However, if a thin film containing a trace amount of a metal catalyst for low temperature crystallization is formed on a substrate, then rather than directly forming on a 7059 glass substrate a thin film containing a trace amount of a metal catalyst for low temperature crystallization, the effect is more notable if a silicon oxide thin film is first formed on the substrate and the thin film containing a trace amount of a metal catalyst for low temperature crystallization is formed over it. One of the reasons that may be imagined for this is that direct contact between the silicon and the metal catalyst for low temperature crystallization is essential for the present low temperature crystallization, and it is thought that in the case of 7059 glass components other than silicon might interfere with the contact or reaction between the silicon and the metal.
Also, roughly the same effect was confirmed when the method used for the trace addition was not formation of a thin film by contact with the top or bottom of the amorphous silicon, but addition of the metal catalyst for low temperature crystallization by ion implantation. Furthermore, it may be added as an impurity at the time of formation of an amorphous silicon film or a non-single crystal silicon film to be crystallized.
Regarding the amount of the metal catalyst for low temperature crystallization, for example in the case of nickel a lower temperature was confirmed with addition of an amount of 1xc3x971015 atoms/cm3 or more, but addition of amounts of 1xc3x971021 atoms/cm3 or more resulted in peaks of Raman spectrum whose shape clearly differed in comparison with simple silicon, and therefore it is thought that the range for actual use is 1xc3x971015 atoms/cm3 xe2x88x925xc3x971019 atoms/cm3. Further, considering that the film will be used as a semiconductor for an active layer of a TFT, this amount must be kept to within 1xc3x971015 atoms/cm3 xe2x88x922xc3x971019 atoms/cm3.
An additional explanation will now be given regarding the assumed mechanism of crystallization when nickel is used as the metal catalyst for low temperature crystallization.
As described above, if no metal catalyst for low temperature crystallization is added, then nuclei are randomly generated from the crystal nuclei on the interface with the substrate, etc., and the crystal growth from these nuclei is likewise random, and it has been reported that relatively oriented crystals obtained in (110) or (111) are obtained depending on the method of preparation, and naturally crystal growth which is roughly uniform over the entire thin film is observed.
First, in order to determine the mechanism, an analysis was made with a DSC (differential scanning calorimeter). An amorphous silicon thin film formed on a substrate by a plasma CVD was placed with the substrate in a sample container, and the temperature was raised at a constant rate. Distinct exothermic peaks were observed at about 700xc2x0 C., and crystallization was observed. Naturally, this temperature shifted when the temperature-raising rate was changed, but when the rate was, for example, 10xc2x0 C./min crystallization commenced at 700.9xc2x0 C. Next, measurements were made with three different temperature-raising rates, and the activation energies of the crystal growth after the initial generation of nuclei were determined by the Ozawa method. This resulted in a value of about 3.04 eV. Also, when the reaction rate equation was determined by fitting with the theoretical curve, it was found to be most easily explainable by a model of disorderly generation of nuclei and growth therefrom, thus confirming the propriety of the model in which nuclei are generated randomly from crystal nuclei on the interface with the substrate, etc., and crystal growth occurs from these nuclei.
Measurements identical to those mentioned above were made with the addition of a metal catalyst for low temperature crystallization, with addition of a trace amount of nickel used as an example. This resulted in initiation of crystallization at 619.9xc2x0 C. with a temperature-raising rate of 10xc2x0 C./min, and the activation energy of the crystal growth determined by a series of these measurements was about 1.87 eV, thus showing numerically as well the readiness of the crystal growth. In addition, the reaction rate equation determined by fitting with the theoretical curve was closer to that of a linear interface rate-determined model, suggesting crystal growth with orientation in a constant direction.
Here, a lowering of the temperature for initiation of crystallization is readily assumed to be an effect of the foreign matter, as described previously, but questions are raised as to the cause of the simultaneous reduction in the activation energy of the crystal growth. In order to investigate the reason therefor, the above method was used to measure the activation energy of recrystallization of an amorphous silicon film prepared by implantation of silicon ions into a non-single crystal silicon film for amorphous state, with no crystallization. As a result, it was found that although crystallization initiation temperature shifted back to the high temperature end, the activation energy of the crystal growth was lowered to about 2.3 eV. Here, considering that almost no hydrogen was present in the amorphous silicon film subjected to the ion implantation, the readiness of the crystal growth may be understood to be rate-determined by the readiness of hydrogen to escape from the interface between the crystal sections and the amorphous sections. As an experimental result to support this hypothesis, the results of TG-DTA (simultaneous thermogravimetry-differential thermal analysis) of amorphous silicon films suggest the very high possibility that initiation of crystallization always occurs directly after pauses in the escape of hydrogen, and thus that the hydrogen impedes crystallization.
Here, a closer look at the reaction between hydrogen and the metal catalyst for low temperature crystallization which is added shows that when any of the candidate metals react with hydrogen to form hydrides, the reaction is exothermic (only palladium exhibits an endothermic reaction according to some documents). This suggests that the metal catalyst for low temperature crystallization is stabilized by bonding with hydrogen, and it is thought that the lowering in temperature is achieved by the following mechanism.
The metal catalyst for low temperature crystallization incorporated into the amorphous silicon forms a direct bond with the silicon. when heat is applied thereto, diffusion of the metal catalyst for low temperature crystallization precedes crystallization to a uniform concentration gradient, but while diffusing it also bonds to hydrogen, and this results in weaker bonds with the silicon which are easily breakable, thus increasing the number of dangling bonds and vacancy in the film. Crystallization necessitates the migration of the silicon atoms, and it is presumed that the increase in dangling bonds and vacancy facilitates this migration, meaning that the preparation for crystallization is made at a lower temperature. Generation of the nuclei occurs afterwards, but the activation energy at this time is reduced by the addition of a trace amount of the metal catalyst for low temperature crystallization. This obviously results from the fact that the addition of the metal catalyst for low temperature crystallization allows the generation of crystals at a lower temperature, and the reason for this is believed to be possibly the effect of the simple metal catalyst for low temperature crystallization as foreign matter or the effect of an alloy of the metal catalyst and silicon.
Also, the generation of the nuclei is almost simultaneous over the entire surface of the area to which the metal catalyst for low temperature crystallization has been added, and consequently the crystal growth follows the mechanism of growth as a surface, in which case the reaction rate equation becomes that of a linear interface rate-determination process and thus matches the results of DSC. The crystal growth from the crystal nuclei progresses afterwards, but since at this time hydrogen is not present at the interface between the crystal sections and the amorphous sections, the rate-determining process changes and the activation energy required for crystal growth is greatly lowered accordingly.
Diffusion of the metal catalyst for low temperature crystallization prior to crystallization is essential in order to explain the above mechanism, but this is assumed to be clear because, in cases of samples which were annealed up until the beginning of crystallization, measurement of the concentration of the metal catalyst for low temperature crystallization by SIMS (secondary ion mass spectrometry) confirmed the presence of the metal catalyst for low temperature crystallization at levels higher than the lower limit even in areas relatively far distant from the area to which the metal catalyst for low temperature crystallization had been directly added.
An additional explanation will now be provided in regard to the crystalline morphology of the crystalline silicon film obtained by the above mentioned addition of a trace amount of a temperature crystallization metal catalyst.
As was briefly mentioned in the explanation of the crystallization mechanism, the added metal has a fairly wide range of diffusion at temperatures below that of crystallization. Consequently, a lowering in the crystallization temperature is also achieved in these areas of diffusion. Furthermore, it has become clear that the morphology of the crystals differs between the area of direct addition and the areas of diffusion. That is, it has been confirmed that the area of direct addition exhibits crystal growth vertically to the substrate, while the surrounding areas of diffusion show growth horizontally to the substrate. It is assumed that this was due to a difference in the initial generation of the nuclei in each case. That is, it may be interpreted that in the area of direct addition the foreign matter serves as the crystal nuclei to produce columnar crystal growth, while in the surrounding areas of diffusion the crystal nuclei must of necessity grow horizontally because of the vertical growth already initiated from the area of direct addition. Hereunder in the present specification such areas of crystal growth. in a horizontal lateral direction on the substrate extending outward from the area of direct addition of the metal catalyst for low temperature crystallization will be referred to as xe2x80x9clateral growthxe2x80x9d regions.
An explanation will now be given regarding the electrical properties of the sections of trace addition of nickel and of the surrounding lateral growth sections, for a case in which nickel is used as the above mentioned metal catalyst for low temperature crystallization. Regarding the electrical properties of the area of trace addition of nickel, the conductivity was about the same value as for a film with no nickel added, or a film heated for a few dozen hours at about 600xc2x0 C. Also, when the activation energy was determined base; on temperature dependence of the conductivity, no behavior seemingly due to the order of the amount of nickel was observed if the amount of nickel was about 1017 atoms/cm3xe2x88x921018 atoms/cm3 as mentioned above. In other words, the experimental facts lead to the supposition that within the above range of concentration the film may be used as the active layer of a TFT, etc.
In contrast, the lateral growth sections had a conductivity which was one order higher in comparison with the area of trace addition of nickel, and this is a high value for crystalline silicon semiconductors. This was thought to be due to the fact that, since the path direction of the current matched the lateral growth direction of the crystals, in the sections between the electrodes through which electrons pass the grain boundary was either small or virtually non-existent, and thus there was no contradiction with the results shown in the transmission electron micrographs. That is, it is conceivable that because the migration of the carrier occurred along the grain boundary of the needle-like- or columnar-growing crystals, a condition for easy migration of the carrier had been created.
In addition, as shown in FIG. 1, when nickel is selectively introduced as a nickel silicide film in the area indicated by 100, and then an amorphous silicon film 104 is formed thereon by the publicly known plasma CVD method and crystallization is induced by heating at 550xc2x0 C. for 4 hours, crystal growth occurs in the nickel-introduced area 100 in a vertical direction to the substrate 101, while in the areas other than 100 horizontal growth occurs parallel to the substrate 101 as shown by the arrows 105. A crystalline silicon film is obtained as a result. When the nickel concentration of such a crystalline silicon film was measured by SIMS, the following discoveries were made.
1. The concentration distribution of the nickel is not so large in the perpendicular thickness direction of the film.
2. The concentration of nickel in the area of direct introduction of the nickel (for example, the area 100 in FIG. 1) is greatly influenced by the conditions of forming the nickel film. In other words, the nickel concentration in that area does not provide very much reproducibility.
3. In the areas with crystal growth parallel to the substrate (areas where nickel had not been directly introduced), the concentration of nickel is about one order or more lower than in the area in which nickel is directly introduced, in 2 above, and the concentration therein is highly reappearing.
4. The background nickel concentration is about 1xc3x971017 cmxe2x88x923, which roughly matches the limit of measurement by SIMS. That is, the background nickel concentration is at or lower than about 1xc3x971017 cmxe2x88x923, the measurable limit of SIMS.
For example, in the area in which nickel is directly introduced and crystal growth occurred vertically with respect to the substrate, if nickel is present at a concentration of about 2xc3x971018 cmxe2x88x923 then the measured nickel concentration will be about one order lower, or about 2xc3x971017, in the areas about 40 xcexcm distant from the nickel-introduced area, in which crystal growth occurred parallel to the substrate, i.e. the areas where lateral growth occurred. The above case will be explained with reference to FIG. 4. FIG. 4 shows the Ni concentration of an area to which Ni was added by plasma treatment (Plasma treated), the Ni concentration of the areas which experienced crystal growth parallel to the substrate (Lateral growth), and the ground level Ni concentration of a-Si. As is clear from FIG. 4, the Ni concentration of the areas which experienced crystal growth parallel to the substrate (Lateral growth) was lower than in the area in which Ni had been directly introduced. Consequently, the areas in which crystal growth occurred parallel to the substrate are more useful for a device.
Since it is very difficult to control the nickel concentration in the silicon film at the area of direct nickel introduction, this nickel concentration fluctuates greatly depending on the conditions of the formation of the nickel film (actually a nickel silicide film). It is thought that this is due to the fact that the nickel concentration in this area (for example, area 100 in FIG. 1) is directly dependent on the film-forming conditions required for severely thin film thicknesses on the order of 20 xc3x85 (because the actual measurement is difficult, this value is deduced from the film-forming rate). As is well known, it is impossible to form satisfactorily uniform films of around 20 xc3x85 over large surface areas using film-formation methods such as a sputtering method. Consequently, it is believed that the poor reproducibility of the forming of the film directly reflects the fluctuation of the nickel concentration in the silicon film. Furthermore, the variations in the nickel concentration also directly affects the properties of a TFT formed using this area in which nickel was directly introduced as the active layer. That is, if a TFT is prepared using this area in which nickel was directly introduced (for example, 100 in FIG. 1), then wide variations will occur in the properties thereof. This is also thought to be a result of the poor reproducibility of the exceedingly thin nickel film.
On the other hand, the nickel concentration in the areas distant from the nickel-introduced area, that is, the areas in which crystal growth from the direct nickel-introduced area occurred horizontally in a direction parallel to the substrate, was generally lower than in the direct nickel-introduced area (as mentioned above, about one order less in sections 40 xcexcm distant), and further a tendency was seen toward smaller variation in that concentration. It is known from experiments that the nickel concentration for an active layer with properties satisfactory for a TFT is from the measuring limit of SIMS or lower (1xc3x971017 cmxe2x88x923 or lower) to about 2xc3x971019 cmxe2x88x923, but in the areas of parallel crystal growth with respect to the substrate, the above given nickel concentrations have been found to be relatively stable regardless of the amount of nickel directly introduced (the nickel concentration of the silicon film 104 in the nickel-introduced area, for example, area 100) In other words, if a TFT is formed using the areas in which crystal growth from the nickel-introduced area occurs parallel to the substrate, then it is possible to obtain a TFT with very good reproducibility.
Furthermore, it has been found to be very easy to select areas with nickel concentrations in the above range, or to adjust the nickel concentrations of certain areas (though not the area indicated by 100 in which nickel has been directly introduced) within the above range. For example, for the selection of areas having specific nickel concentrations, a specific nickel concentration may be attained by setting the distance from the nickel-introduced area. However, in this case the required crystallinity of the silicon film must be obtained.
Also, the nickel concentration in the areas of crystal growth parallel to the substrate may be controlled by controlling the conditions of crystallization (mainly the heating time and heating temperature), and this control is much easier than control of the nickel concentration in the area in which nickel has been directly introduced.
Thus, the areas of crystal growth parallel to the substrate, from the area to which the catalyst element has been added for crystallization, that is, the areas of the lateral growth, are useful for employment in semiconductor devices because
(1) the orientation of the crystals may be positively used, and the carrier with a high degree of mobility may be used;
(2) the areas of low concentration of the catalyst material for crystallization may be used;
(3) the areas in (2) above have good reproducibility; and
(4) it is possible to easily control the concentration of the catalyst material for crystallization.
Finally, an explanation will be given regarding methods of applying the various properties mentioned above to a TFT. Here, the field of application of the TFT is assumed to be an active matrix-type liquid crystal display which uses the TFT as a driver for a picture element.
As described previously, it is important to minimize the shrinkage of the glass substrates in recent large-screen active matrix-type liquid crystal displays, and by using the process of adding a trace amount of a metal catalyst for low temperature crystallization according to the present invention the crystallization is possible at a sufficiently low temperature compared to the distortion point of glass, and thus it is a particularly suitable method. By following the present invention, the section using conventional amorphous silicon may easily be replaced by crystalline silicon by adding a trace amount of a metal catalyst for low temperature crystallization thereto and conducting crystallization at about 500-550xc2x0 C. for about 4 hours. Obviously, some variations will be required to adapt to specific rules of design, etc., but the present invention may be carried out satisfactorily with the devices and the processes of the prior art, and thus its advantages are thought to be considerable.
Furthermore, according to the present invention, TFTs used in picture elements and TFTs which make up the drivers of peripheral circuits may be made as separate types by employing crystal morphology adapted to their respective desired properties, and they offer particular advantages when applied to active-type liquid crystal displays. TFTs used in picture elements do not demand so much mobility, and instead their major advantage is a low off-state current. Here, when employing the present invention, a trace amount of a metal catalyst for low temperature crystallization may be directly added to the area which is to become the TFT for a picture element, to cause vertical growth of crystals, and to form a number of grain boundaries with respect to a channel direction resulting in a lower off-state current. In contrast, a TFT which is to form the driver of a peripheral circuit requires a very high degree of mobility if its future application in a workstation or the like is being considered.
Here, if the present invention is applied, trace amounts of a metal catalyst for low temperature crystallization are added near the TFT which is to form the driver of the peripheral circuit, and from there growth of crystals is caused in a single direction (parallel to the substrate), and by aligning the direction of the crystal growth with the path direction of the channel current, it is possible to produce a TFT with a very high degree of mobility.
Furthermore, there are known devices comprising a glass substrate on which are integrated sensors for handling picture information or optical signals. For example, integrated image sensors and the like are known. Such devices, if they are to be used for the detection of visible light, preferably use amorphous silicon (a-Si) from the point of view of spectral sensitivity. However, since driving circuit sections must have switching elements for which high-speed operation is demanded, the element of a driving circuit section, for example the TFT, is preferably not made of an amorphous silicon film because of its insufficient mobility. However, it is useful to use the above mentioned high-mobility TFT. For example, a photo diode or photo transistor employing an amorphous silicon film is formed in the sensor section, and the peripheral circuit sections form transistors using the crystalline silicon film according to the present invention. These circuits may also be integrated on the same substrate (for example, a glass substrate) with composite structures.
In other words, by employing the present invention, it is possible to create areas with crystalline silicon films and areas with amorphous silicon films in prescribed areas, and by using the areas with crystalline silicon films in which crystal growth has occurred laterally, it is possible to form a device in which the carrier may migrate at a high speed. This utility is not limited to liquid crystal displays and sensors, and may be applied to a wide variety of semiconductor devices integrated onto substrates. That is, it may be used in transistors and diodes which employ thin-film semiconductors on substrates, as well as devices containing integrated resistors and capacitors.
By construction of an active layer of a TFT or the like by growing crystals in a needle-like or columnar manner parallel with respect to a substrate from an area into which has been introduced a trace element selected mainly from the Group VIII elements which have catalytic effects for crystallization of silicon, it is possible to use as the active layer the areas with lower concentrations of the trace element than the area into which it has been introduced, and thus it is possible to obtain a device which is not affected by the above mentioned trace element.
Furthermore, during the formation of the device, the carrier flow may be set to be aligned with the direction of the crystal growth of the crystalline silicon film in a needle-like or columnar manner, in order to improve the properties of the device. In addition, since the concentration of the above mentioned trace element in these areas is either low or may be easily controlled, it is possible to obtain a device with the necessary properties and with good reproducibility.